In what he describes as ''the biggest victory of my career,'' Mel Purcell defeated Ivan Lendl in the quarterfinals of the US Pro Tennis Championships here last weekend. Purcell, ranked 30th in the world amazed the crowd, and probably himself, by knocking the Czech star out of the competition 2-6,6-3,6-2.

With players like Lendl, Jimmy Connors, John McEnroe, Bjorn Borg, and Guillermo Vilas garnering most of the headlines, it's easy to forget the hundreds of other pros waiting for their chance to break into the winner's circle. Purcell's decisive victory over Lendl moved him a big step closer to that goal.

Purcell, who just turned 23 but looks much younger on the court, grew up in Murray, Ky., where his father, Bennie, is the tennis coach at Murray State University.

A self-described natural athlete, Mel says he has been in love with tennis from the age of five. He also played baseball and basketball at first, but lost interest in these sports and has devoted himself to tennis from age 12. Under the tutelage of his dad, who continues as his coach, he progressed rapidly through high school and college (he attended Memphis State and the University of Tennessee), then left school after his junior year to turn pro.

He was the 1980 pro Rookie of the Year, and in the past two years he's won many matches and a few minor tournaments in cities like Tampa and Tel Aviv. But a major victory has eluded him. He thinks his victory over Lendl will prove to be a turning point.

Despite the rigors of the tour, Purcell makes every effort to practice several hours per day. He also tries to maintain a ''normal'' lifestyle. But this is difficult: he will spend 38 weeks this year in hotels, living out of suitcases, as he competes in 32 tournaments

Purcell (no relation to the writer) thinks quickness and athletic ability are his biggest assets. Those combined with a lightning quick forehand, he says, help compensate for a so-so backhand and an inconsistent serve.

Clay courts are his best surface, and he says the heat is to his advantage. He had the opportunity to demonstrate this at the US Pro championships, where court temperature soared above 110 degrees on several days.

Purcell, who sometimes runs around his backhand to hit a stronger shot off the forehand, places the ball well, and runs his opponents back and forth across the court. He never lets up. Lendl, who spent two hours chasing Purcell's shots was asked if the heat beat him. ''No,'' he replied, ''Purcell did.''

But Purcell says lack of confidence has been his greatest problem. Thus beating Lendl, who has already won more than $1 million this year and is one of the game's current titans, changed his outlook.

''I used to have no confidence that I could win in the finals, but tonight I really thought I could,'' he said of his match against Vilas. And although the top-seeded Argentine veteran eventually won (6-4, 6-0), Purcell's play early in the match displayed a good deal of poise and professional maturity.

Purcell tries to project the image of the ''good ol' boy just off the farm,'' but that's not really the case. He may have been naive when he started playing professionally, but when he steps onto the court he plays as hard and intelligently as anyone on the tour.

Mel, whose $16,000 second prize here raised his earnings for the year to around $105,000, says he's in the pro game mostly for the money. His playing indicates otherwise. His game is fast, athletic, and competitive, mixed in with just enough ''showboating'' - playing to the audience - to keep a large portion of the crowd pulling for him.

But it's not his showboat routines, his desire to win a major tournament, or the money that sends a spark of excitement through the crowd when Purcell plays. It's his obvious enthusiasm, his love for the game.

As Vilas said, ''You can't let this become a job. Otherwise it gets boring. The only thing that keeps you going is your love for the game.'' Clearly Purcell feels this too. His enthusiasm and the energy he puts out lets you know that here is someone who likes to play the game, someone who is having fun.

Even after his loss to Vilas, his outlook was bright. He didn't view the outcome with sorrow or resignation. He seemed pleased to have done so well. Mel Purcell, whose present goal is to get into the top 20, looked ready to try harder next time. He's sure he'll be back.

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